(1) Field of the Invention
This invention relates generally to ultrasound detection and imaging and, more particularly, to novel compositions, methods and devices for detecting a change in ultrasound reflectivity based upon changing the temperature of a temperature-dependent contrast agent bound to the target.
(2) Description of the Related Art
Molecular imaging can enhance the utility of traditional clinical imaging by allowing specific detection of molecular markers in tissues using site-targeted contrast agents (Weissleder, Radiology 212:609–614, 1999). Three approaches to site-targeted ultrasonic agents have been reported and these are based upon the use of liposomes (Alkan-Onyuksel et al., J. Pharm. Sci 85:486–490, 1996; Demos et al., J. Pharm. Sci. 86:167–171, 1997; Demos et al., J. Am. Col. Cardiol. 33:867–875, 1999), the use of microbubbles (Mattrey et al, Am. J. Cardiol. 54:206–210, 1984; Unger et al., Am. J. Cardiol. 81:58G–61G, 1998; Villanueva et al, Circulation 98:1–5, 1998; Klibanov et al, Acad. Radiol. 5S243–S246, 1998) or the use of nano-emulsions (Lanza et al, Circulation 94:3334–3340, 1996; Lanza et al, J. Acoust. Soc. Am. 104:3665–3672, 1998; Lanza et al, Ultrasound Med. Biol. 23: 863–870, 1997). Liposomes are spherical bimembrane vesicles produced spontaneously by phospholipids in water. Multilamellar lipid bilayers produced through a dehydration-rehydration process can form internal vesicles within a liposome and lead to increased acoustic reflectance (Alkan-Onyuksel et al., 1996 supra; Demos et al., 1997, supra; Demos et al., 1999, supra). In the second approach, microbubbles have been proposed for site-targeted modalities in addition to their perfusion applications. Microbubbles have been targeted towards thrombi (Unger et al., 1998 supra; Lanza et al., Ultrasound. Med. Biol. 23: 863–870, 1997), avidin-coated petri dish (Klibanov et al, 1998, supra) and activated endothelial cells (Villanueva et al, 1998, supra). Other investigators have examined the interaction of thrombus with site targeted agents. In particular, Unger et al. has observed successful binding of MRX-408, a bubble-based contrast agent, both in vitro and in vivo (Unger et al., 1998, supra).
The site-targeted nano-emulsions are nongaseous acoustic contrast agents made up of lipid-encapsulated liquid perfluorocarbon nanoparticules (see Lanza et al., U.S. Pat. Nos. 5,690,907; 5,780,010; and 5,958,371). The nanoparticles are approximately 250 nm in diameter. Perfluorocarbon nanopatriculate emulsions have been shown to provide substantial acoustic contrast when targeted towards in vitro and in vivo thrombi preparations (Lanza et al., 1998, supra; Lanza et al., 1997, supra).
One of the challenges confronting the use of site-targeted contrast agents is the sensitive detection and differentiation of the particles from the surrounding soft tissue. Detection of pathological changes on or near vascular surfaces may be compromised because the targeted substrate itself is echogenic or the signal from that surface may be somewhat view or angle dependent. Imaging techniques have been developed in attempts to solve this issue. Second harmonic or harmonic and power harmonic Doppler imaging has been used to allow differentiation of microbubbles in circulation from tissue (see Burns et al. Clinical Radiol. 51:50–55, 1996; Kasprzak et al, Am. J. Cardiol. 83:211–217, 1999; Senior et al, Am. Heart J. 139:245–251, 2000; Spencer et al, J. Am. Soc. Echo. 13:131–138, 2000). However, soft tissue also exhibits a second harmonic backscattered signal. Furthermore, the contrast agent may manifest velocities too slow for the sensitivity of Doppler techniques. Unlike the resonance phenomenon responsible for enhanced backscatter cross section in microbubbles, the mechanism for increased reflection enhancement from the site-targeted nanoparticle emulsions has been reported to be due to acoustic impedance mismatch at the surface where the particles bind (Lanza et al., 1998, supra). Thus, although site-targeted acoustic contrast agents and, in particular, the nano-emulsion contrast agents have been used as contrast agents, the development of approaches that produce a greater degree of contrast could potentially provide further sensitivity for ultrasound molecular imaging systems.
Perfluorocarbon liquids are known to transmit ultrasound at low velocities (Lagemann et al. J. Am. Chem Soc. 70: 2994–2996, 1948; Gupta, Acustica 42:273–277, 1979). The low ultrasound velocities through these substances have been shown to be temperature dependent in that the ultrasound velocity is decreased in a linear manner with increasing temperature (Narayana et al., Acoustics Letters 9:137–143, 1986). This observation was reported to be potentially applicable to the development of acoustic lenses (Id.). Nevertheless, the temperature-dependence of ultrasound velocity in perfluorocarbon liquids has not, heretofore, been suggested to have any applicability in ultrasound imaging systems.
Ultrasound energy has been applied in site-targeted contrast agents in ultrasound imaging methods as noted above. Much of this earlier work was directed to molecular imaging so that only low level ultrasound energy was used and no change in temperature of the targeted surface was reported to occur. In microbubble ultrasound imaging systems, sufficient energy has been applied to a liquid precursor substance to form gaseous microbubbles. (Lohrmann et al., U.S. Pat. No. 5,536,489; Unger, U.S. Pat. No. 5,542,935). One suggested approach has been to apply the energy to produce the phase shift in vivo. In such approaches, temperature changes would serve to convert the gaseous precursor to the gaseous microbubbles and none of these earlier studies disclosed or suggested changing temperature of an ultrasound contrast agent which remains in the liquid state or using the change in temperature of a nongaseous contrast agent as a basis for enhancing ultrasound detection.
Thus, there remains a continuing need for developing approaches that produce an enhanced degree of contrast and provide further sensitivity for ultrasound molecular imaging systems.